232 



EXTINCT OEDERS OF REPTILES. 



The teeth oi Iguanodon (fig. 577) present a singularly close 

 resemblance in shape to those of the comparatively pigmy 

 Iguanas of the present day. Their crown is obtusely sub- 

 triangular, with longitudinal ridges, and having the surface 



-Teeth ut' Iguanodon MantellU. Wealdeii. 



of the enamel crenated on one or both sides. They present 

 the extraordinary feature that the crown became worn down 

 flat by mastication, showing that Iguanodon employed the 

 teeth in the actual trituration of the vegetable matter on 

 which it fed. The front portion of the jaws seems to have 

 been toothless and beak-like, and the symphysis of the man- 

 dible is hollowed out above ; the purpose served by this, 

 according to the views of Professor Owen, being to facilitate 

 the easy protrusion and retraction of a long muscular and 

 prehensile tongue, employed by the animal in stripping off 

 the foliage of trees. In the Cretaceous deposits of North 

 America the genus Iguanodon is represented by the nearly- 

 allied Hadrosaurus. Iguanodon is not known to have pos- 

 sessed any exoskeletal structures capable of preservation in 

 the fossil state ; but the Hylmosaurus of the Wealden pos- 

 sessed bony dermal scutes, prolonged along the middle line 

 of the back into a row of enormous spines. 



The gigantic Cetiosaurus of the Oolitic and Cretaceous 

 rocks was originally placed amongst the Crocodilia ; but the 



